1. Field of the Invention
The present invention relates to an image encoding apparatus for encoding information contained in a video (time-varying image) so as to reduce an amount of information contained in the video signal.
2. Description of the Related Art
Recently, with developments of high-speed digital signal processing and large scale integration (LSI) techniques therefor, with the progress of image processing techniques, making efficient use of image information is expected. In particular, in the field of communication, active studies are conducted for a high-functional network enabling which transmits image information efficiently. For B-ISDN (Broadband-Integrated Services Digital Network), which is a wide-band communication network of the next generation, it is expected that very high quality image transmission service will be realized supplied.
In general, it is unpractical to process a video signal as it is, since too much information is contained in an image information is too much. However, since much redundancy is involved in its information the amount of information can be reduced by removing the redundancy. Because of this, a video compression encoding technique is very important for efficiently processing the video information, and active studies are conducted concerning a video encoding system therefor.
A video signal comprises temporal information about movement, change, etc. and spatial information about the contents of a piece of video frame, both of which include redundancy. The amount of information can be greatly reduced by reducing time redundancy since successive video frames have much correlation, particularly in a video (time-varying) signal. A motion compensated interframe prediction encoding system is widely used: in general as an encoding system by means of such time correlation.
An example of a known image encoding apparatus is described below.
An inputted video signal is sent to a motion compensated predictor in non-duplicate rectangular blocks by each video frame. The motion compensated predictor effects a motion compensated prediction between the inputted video signal and a video signal inputted prior to the inputted video signal and stored in the frame memory to detect and output a motion vector.
A block judged to be the most appropriate is read out as a predicted value from the frame memory, resulting from the above motion compensated prediction.
A difference operator obtains the difference between the predicted value and the inputted image signal to output a prediction error signal which is sent to an error encoder. The error encoder effects appropriate encoding such as DCT (Discrete Cosline Transform) encoding by means of a correlation of a prediction error signal inside a video frame, resulting in an encoded signal to be outputted to the outside of the device together with a motion vector from the motion compensated predictor.
On the other hand, the encoding signal is locally decoded in the error decoder to output a prediction error decoding signal which is added at the adder again to a predicted value read out from the frame memory to obtain a video signal reproduced after encoded.
This reproduced video signal is sent to and stored in the frame memory. Upon a completion of encoding a video frame, the frame memory stores a locally decoded and reproduced video signal of the video frame which is used as a predicted value of motion compensated prediction of the following video frame to be encoded.
However, in the above-mentioned known image encoding apparatus in which encoded video frames are used one after another for a prediction encoding of the following video frames, an interframe prediction may be prevented by strong correlation between information involved in a video frame to be subsequently encoded and information involved in the following video frame to be encoded, resulting in a problem of uncertainty about the effects of motion compensated prediction encoding.